Hierarchical synchronization method

ABSTRACT

A hierarchical synchronization method and equipment for a node in a telecommunications system employing message-based synchronization and comprising a plurality of nodes interconnected by transmission lines. Signals containing synchronization messages having a synchronization signature are interchanged with neighboring nodes, the synchronization signature indicating the priority of the respective signal in the internal synchronization hierarchy of the telecommunications system and containing a distance parameter representing a distance to a master source of synchronization and a master node parameter indicating an original synchronization source. The node synchronizes with the one of signals received from the neighboring nodes which has the highest priority in the internal synchronization hierarchy. A synchronization list is maintained, the list comprising, on the highest level, the synchronization signature of the signal with which the node is synchronized. The distance parameter and the master node parameter in the synchronization signature on the highest level in the priority list are monitored. The method includes changing over from a normal state to a predetermined state in order to prevent synchronization with signals having faulty synchronization signatures when the value of the distance parameter increases a predetermined number of times within a predetermined monitoring period while the master node parameter remains unchanged within the monitoring period.

This application claims benefit of international applicationPCT/F195/00110, filed Feb. 28, 1995.

1. Field of the Invention

The invention relates to a hierarchical synchronization method, themethod being used in a telecommunications system employing message-basedsynchronization and comprising a plurality of nodes interconnected bytransmission lines. The invention also relates to a node equipment fortelecommunications systems employing message-based synchronization andcomprising a plurality of nodes interconnected by transmission linesrealizing such a method.

2. Background

In this presentation, the junction points of the transmission lines in asystem are called nodes. A node may be any device or equipment capableof affecting clock synchronization, such as a branching orcross-connection means.

Nodes in a system utilizing message-based synchronization areinterconnected by transmission lines which the nodes use for datatransmission. These lines also forward the clock frequency of thetransmitting party to the receiving party. Each node selects as thesource of its own clock frequency either the frequency of a signal froma neighbouring node, the frequency of its own internal clock source, ora frequency brought into the node from an external clock source througha separate synchronization input. In order that all nodes in the systemwould operate at the same clock frequency, the aim is usually to makethe system synchronize itself with a single clock source called a mastersource. All system nodes connected directly to the selected mastersource are thus synchronized with the master source while nodesconnected to the nodes adjacent to the master source but not directlyconnected to the master source are synchronized with these adjacentnodes. Accordingly, each node at a greater distance from the mastersource synchronizes itself with a node one node spacing closer to themaster source.

In order that the above-described synchronization hierarchy could beestablished within the system, the system nodes interchangesynchronization messages. These messages contain information by means ofwhich individual nodes are able to select a timing source. The systemnodes are prioritized and the system tends to synchronize itself withthe clock frequency of a node having the highest level of priority.Normally each priority level is assigned to a single system node.Synchronization messages normally contain information about the originof the clock frequency of the node transmitting the message and thepriority of the node as well as a value describing the quality of theclock signal. Accordingly, a neighbouring clock frequency whichoriginates from a desired node and which is of the highest quality canbe selected by an individual node as the source of its own clockfrequency. At the system start-up each node selects its own internalclock source as the source of its clock frequency as it has not yetprocessed any incoming synchronization messages. After the node hasprocessed the first incoming synchronization messages, it selects theclock frequency of a neighbouring node having the highest level ofpriority as the source of its clock frequency. After all messages havebeen distributed over the system and the system has achieved a stablestate as far as synchronization is concerned, the system has beensynchronized hierarchically with the clock frequency of the mastersource.

FIG. 1 shows a system utilizing message-based synchronization in astabilized situation. Priorities assigned to the nodes are indicated bynumbers within the circles representing the nodes. The smaller thenumber, the higher the priority of the node. Synchronization messagestransmitted by a node n (n=1 . . . 6) are indicated by the referenceMSG_(n). Synchronization messages transmitted by different nodes usuallydiffer from each other and depend on the applied message-basedsynchronization method. The distribution of the clock frequency from themaster clock (node 1) to the other system nodes is illustrated by solidlines. Internodal connections drawn by broken lines are not used in anormal situation for system synchronization, but they are available inchange situations.

Message-based synchronization is based on a simple principle that theuser defines the synchronization hierarchy of the system nodes byassigning each node a dedicated signature indicating the hierarchicallevel of the node and the system synchronizes itself with the definedmaster clock independently by utilizing, if required, all existinginternodal connections. If the connection to the master clock breaks,and no alternative connection exists, or if the master clock fails, thesystem synchronizes itself with a node of the next highest level ofhierarchy. Response to the change in synchronization takes place bymessage interchange between nodes.

A message-based synchronization method of the type disclosed above isdescribed e.g. in U.S. Pat. No. 2,986,723, which is referred to for amore detailed description. Messages used in this type of knownmessage-based synchronization method (SOMS) are described in greaterdetail below in connection with FIGS. 2 and 3, since the methodaccording to the present invention is applicable for example in thistype of system.

In a system utilizing message-based synchronization, an outdatedsynchronization signature having the wrong synchronization status maykeep on circulating in the network in failure situations despitecheckings, and in circulating in the network it will soon bring aboutdisorder in the synchronization of the entire network.

The known synchronization methods described in the literature are basedon the idea that no outdated messages remain in the network; however,these descriptions contain no protection procedures to ensure thissituation. The method according to the aforementioned U.S. Pat. No.2,986,723 suggests that the old messages be eliminated from the networkby utilizing time periods during which no new messages are accepted, buta certain state is retained so that the old signatures disappear fromthe network. If old signatures still remain in the network after thesetime periods have expired, no methods with which the system couldrecover from this situation have been provided.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a simple method bymeans of which it is possible to detect, as reliably as possible, anoutdated synchronization signature circulating in the network. This aimis achieved with the method according to the invention which ischaracterized by what is described in the characterizing portion of theappended claim 1. The node equipment according to the invention is inturn characterized by what is described in the characterizing portion ofthe appended claim 7. The arrangement according to the invention may beused as the only means of detection or as additional backup for othermethods, so that the old messages can be eliminated with the greatestpossible certainty.

The idea of the invention is to monitor both the distance parameterwhich is contained in the synchronization signature of a source on thetop of the node's priority list and which describes the distance to themaster source of timing, and the parameter contained in thesynchronization signature and describing the original synchronizationsource, and to enter into a predetermined state in order to prevent theselection of faulty synchronization signatures, if the value describingthe distance increases often enough during a predetermined short periodof time while the parameter describing the original synchronizationsource remains the same, however, during the same short time period. Thesolution according to the invention can thus be utilized in networkswhere the synchronization signature comprises in some form informationabout the distance to the master clock of the network and about theoriginal timing source, and where the node utilizes this information asa criterion for selection (possibly together with the quality parameterdescribing the original timing source). Methods fulfilling theserequirements include for example the aforementioned SOMS method and themethod disclosed in Information to ISO/IEC CD 11573 -- InformationTechnology -- Telecommunication and Information Exchange between Systems-- Synchronization Methods and technical requirements for PISNs, ISO/IECJTC1/SC6/WG6 N, July 1992 (reference 1).

By means of the arrangement according to the invention it is possible todetect the occurrence of disorder in synchronization already at an earlystage and to recover from this situation before the synchronization ofthe entire network gets in disorder.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention and its preferred embodiments will bedescribed in greater detail with reference to the examples according toFIGS. 2 to 8 in the accompanying drawings, in which

FIG. 1 shows a system employing message-based synchronization when thesystem is in synchronization with the clock frequency of a mastersource,

FIG. 2 shows a network employing self-organizing master-slavesynchronization (SOMS) in an initial state,

FIG. 3 shows the network of FIG. 2 in a stable state,

FIG. 4 illustrates the resynchronization of the network of FIG. 3 whenthe master node has failed.

FIG. 5 illustrates the resynchronization of the network of FIG. 3 when aconnection between two nodes has failed,

FIG. 6 is a flow chart illustrating the main steps of the methodaccording to the invention,

FIG. 7 shows means provided in each individual node for realizing themethod according to the invention, and

FIG. 8 is a flow chart illustrating a more detailed embodiment of themethod according to the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENT

FIG. 2 illustrates a system employing the aforementioned self-organizingmaster-slave synchronization (SOMS), which is a message-basedsynchronization method known per se. In this specific case, the systemcomprises five nodes (or devices) which are indicated by the referencenumerals 1 . . . 5 according to their level of hierarchy. (Each numberforms the SOMS address of the node, and the master node of the networkhas the smallest SOMS address.) The nodes interchange messagescontaining such SOMS addresses. In this way the nodes are able toidentify each other by means of the address numbers and establish asynchronization hierarchy so that the whole network can synchronizeitself with the master node.

As mentioned above, messages transmitted continually in the network aredependent on the applied message-based synchronization method. Inaddition, the messages are specific for each transmitting node. In theSOMS network a synchronization message contains three different parts: aframe structure, signature and check sum. The SOMS signature is the mostimportant part of the SOMS message. It comprises three consecutivenumbers D1 to D3:

D1 is the origin of the synchronization frequency of a node transmittinga SOMS message, i.e. the SOMS address of a node appearing as a masternode to the transmitting node.

D2 is a parameter describing the quality of a connection, and it istypically a distance to a node indicated by D1. The distance is given asthe number of intermediate nodes.

D3 is the SOMS address of a transmitting node.

The parameters central to the present invention are the distanceparameter D2 and the parameter D1 describing the originalsynchronization source. (It should be mentioned that for example in thesystem disclosed in the aforementioned reference 1, the parameterdescribing the original timing source is incorporated into the sameparameter with the distance parameter.)

Each node (or device) compares continuously incoming SOMS signatureswith each other and selects the smallest amongst them. In the signaturethe different parts D1, D2 and D3 are combined into a single number byplacing them in succession (D1D2D3) (for the sake of clarity, a dashwill be inserted between the different parts in the text below asfollows: D1-D2-D3). Accordingly, a primary criterion for the selectionof the smallest address is the SOMS address (D1) of a node appearing asthe master node to the preceding nodes, i.e. the node tends to besynchronized with a signal having a frequency originally derived from anode with the smallest possible address. In a stable situation, thewhole network is thus synchronized with the same master node (as themaster node of the whole network has the smallest SOMS address).

If two or more of the incoming signals are synchronized with the samemaster node, the one arriving over the shortest path (D2) is selected.The last criterion for selection is the SOMS address (D3) of the nodetransmitting the SOMS message, which is used for the selection if theincoming signals cannot be distinguished from each other in any otherway.

After the node has accepted one of the neighbouring nodes as its newsynchronization source on the basis of an incoming SOMS signature, ithas to regenerate its own (outgoing) SOMS signature. The new SOMSsignature can be derived from the selected smallest SOMS signature asfollows: the first part (D1) is left intact; the second part (D2) isincremented by one, and the third part (D3) is replaced with the node'sown SOMS address.

Each node also has its own internal SOMS signature X-O-X, where X is theSOMS address of the node. If none of the incoming SOMS messages containsa signature smaller than the internal signature, the node uses its owninternal oscillator or possibly a separate synchronization input as thesource of its clock frequency. Of course, the outgoing SOMS messagethereby employs the internal SOMS signature.

The nodes transmit continuously SOMS messages in all directions in orderthat any changed data in the SOMS signatures would be distributed asrapidly as possible and that they would know the current operatingcondition of neighbouring nodes. The SOMS signatures cannot be comparedwith each other until the incoming SOMS messages have been accepted andthe SOMS signatures have been extracted from the messages.

When the first SOMS message is received from a specific transmissionline, the SOMS signature contained therein is accepted immediately forcomparison if the message is faultless. When the incoming transmissionline has an accepted SOMS signature and faultless messages containingthe same signature are received continuously, the situation remainsunchanged. If the SOMS message is found to be faulty, the current SOMSsignature is retained until (for example) three successive faulty SOMSmessages have been received. At this stage the old SOMS signature is nolonger accepted for comparison. Waiting for three successive SOMSmessages aims at eliminating temporary disturbances.

If no SOMS message is received from the line and there is no linefailure, the current SOMS signature is rejected only after a period oftime corresponding to (for example) three successive SOMS messages. Ifthe line fails totally, the SOMS signature is rejected immediately. Ifno appropriate SOMS signature is available for comparison due todisturbances in the incoming signal, the SOMS signature of thetransmission line is rejected. A constant-value signature where allparts (D1, D2, D3) have their maximum value (MAX-MAX-MAX) is therebyused in the comparison as the SOMS signature of this incomingtransmission line.

When a new changed SOMS signature is detected in an incoming SOMSmessage, it is accepted immediately for comparison, if the message isfaultless. In this way there will be no unnecessary delays in networkchanges.

Initially each node employs its own internal synchronization source, andtransmits its own internal SOMS signature X-O-X to the other nodes. Thissignature is also compared with incoming SOMS signatures. If none of theincoming signatures is smaller than the internal signature, the nodecontinues to use its own internal timing.

In FIG. 2, the SOMS network is shown in an initial state when none ofthe nodes (or devices) has yet processed any one of the incoming SOMSmessages. In all nodes, the highest priority is assigned to the internalSOMS signature of the node as no other signatures have yet beenprocessed. In FIG. 2, the SOMS signatures are indicated beside each nodeto which they are transmitted, and the selected signature is framed (inthe initial situation shown in FIG. 2 all nodes employ their internaltiming source). Lines used in synchronization are drawn by a continuousline and standby lines are drawn by a broken line (in the initialsituation shown in FIG. 2, all lines are standby lines).

When the nodes start to process the incoming SOMS messages, node 1retains the use of the internal timing, nodes 2 and 4 synchronizethemselves with node 1 on the basis of the signature 1-0-1, node 3 issynchronized with node 2 (2-0-2), and node 5 with node 3 (3-0-3). At thesame time the nodes generate their own new SOMS signatures as describedabove and provide their outgoing SOMS message with the new signature.The network in a stable situation is shown in FIG. 3. All nodes havesynchronized with the master node 1 over the shortest possible path.

If the smallest one of the SOMS signatures entering the node changes oris lost totally when the connection fails, the node selects a newsynchronization direction on the basis of the second smallest SOMSsignature. Prior to this, however, the node is forced to change over tointernal timing, which it retains for a preset time period in order forany faulty SOMS signatures occurring in the network to be eliminated.For instance, if node 1 in the situation of FIG. 3 should fail, nodes 2and 4 would no longer receive the signature 1-0-1, with which they weresynchronized. If they now accepted immediately the second smallest SOMSsignature, the network would no longer be synchronized with a singlemaster node but a synchronization loop would result. When node 1 fails,node 2 still receives the signatures 1-1-4 and 1-2-3, and node 4receives the signatures 1-1-2 and 1-2-5, as nodes 3 and 5 have not yetresponded to the changed situation. If the second smallest signatureswere accepted immediately, node 2 would be synchronized with node 4, andnode 4 with node 2. This situation is prevented by the above-mentionedforced state of internal timing, in which the nodes start to use theirown internal timing source and transmit their own internal SOMSsignature (X-O-X). Nodes which were synchronized with the node now inthe state of internal timing detect that a change has occurred in thenetwork and that the SOMS message on which the former synchronizationwas based is no longer valid as it has been changed into the internalSOMS message of the neighbouring node. As a consequence, the nodes alsoenter into the forced state of internal timing for a preset time period.

If the master node fails in the case of FIG. 3, the nodes 2 and 4 areimmediately forced to enter into the state of internal timing when theylose the incoming SOMS signature 1-0-1. When the nodes 3 and 5 detectthe change that has taken place in the nodes 2 and 4, they are alsoforced to enter into the state of internal timing. When node 2 revertsto the normal state, it receives the internal SOMS signatures (3-0-3 and4-0-4) from the nodes 3 and 4 and retains the use of the internal timingas the SOMS signatures received from outside are not smaller than itsown internal signature (2-0-2). Node 4 is then synchronized with node 2.After having stabilized, the network is in the state shown in FIG. 4,where node 2 is the new master node of the network. If for example onlythe connection between the nodes 1 and 2 breaks (FIG. 5), only node 2 isforced into the state of internal timing. On reverting to the normalstate it synchronizes itself with node 4 having a connection to themaster node of the network. After the stabilization of the entirenetwork, the synchronization still originates from node 1 despite thebreak. This is illustrated in FIG. 5.

As mentioned above, an outdated synchronization signature having thewrong synchronization status may keep on circulating in the network forexample due to a failure situation in a network of the type describedabove, and in circulating in the network the signature soon causes adisorder in the synchronization of the entire network. By means of themethod according to the invention, such a circulating signature isdetected and procedures are started in order to revert to the normalstate of the network.

FIG. 6 is a flow chart illustrating the main steps of the methodaccording to the invention. Whenever a node has selected a newsynchronization source on the basis of a new synchronization signature(step 61), the circulation situation of the synchronization signature ischecked according to the invention (step 62). If a possible circulationsituation is detected (step 63), recovery measures are started (step64). Otherwise normal operation is continued (step 65) and the situationis checked again after the node has selected a new incomingsynchronization signature on the basis of which a new outgoingsynchronization signature should be formed.

Figure. 7 shows means provided in each node for realizing the methodaccording to the invention. The general structure of the node comprisesseveral parallel interface units IU1, IU2 . . . IUN each of which isconnected to a neighbouring node, and a control unit CU which is sharedby all interface units and which is the location of decision-makingconcerning the synchronization. The figure shows two transmissionconnections A and B between a system node and neighbouring nodes, bothconnections being connected to their own interface unit. Thetransmission connections are typically for example PCM lines of 2 Mbit/scomplying with the CCITT recommendations G.703 and G.704, or SDH linesaccording to the recommendations G.708 and G.709. Synchronizationmessages can be transmitted in different ways in such signals; oneexample is disclosed in the co-pending Finnish Patent Application 940926which also describes the general structural models of the node.

Each transmission connection is connected to a signal transmission andreception means 13a and 13b, respectively, which process the physicalsignal. The means 13a and 13b forward the synchronization message to anassociated synchronization message transmission and reception means 16aand 16b, respectively. The transmission and reception means 16a and 16bfor example check whether the message is faultless and forward themessage to a centralized node synchronization decision means 23 having arespective input connected to the output of the respective transmissionand reception means 16a, 16b. The signal transmission and receptionmeans 13a and 13b also supervise the quality of the received signal andstore information thereon into interface-specific fault databases 24aand 24b, respectively. The synchronization message transmission andreception means 16a obtains fault data from the database 24a and thetransmission and reception means 16b from the fault database 24b,respectively. The signal transmission and reception means monitorfailures/changes in the connection in a manner known per se.

The decision means 23 of the control unit CU compares the messages andstores them in a memory 21, e.g. in priority order so that the selectedsynchronization signature always has the highest status. The decisionmeans also receives the fault data of a signal from the correspondinginterface unit in the form of a synchronization message or as separatefault data. When the decision means judges from the supplied data thatthe node has to enter into the standard state for the preset timeperiod, it selects the source of its timing as defined in the appliedsynchronization method for this kind of situation; it applies anappropriate synchronization signature to the interface-specificsynchronization message transmission and reception means 16a and 16bfrom a memory 22 (where it generates an outgoing signature used in eachparticular case), and starts a timer means 25. The node informs theneighbouring nodes about the change that has occurred by transmittingthe new signature. When the timer means 25 indicates that the presettime period K has expired, the decision means 23 is again allowed toselect the source of timing according to a normal procedure.

When the decision means 23 has selected a new synchronization signature(on the basis of which it (possibly) forms later the node outgoingsignature), it also starts a second timer means 26 related to the methodaccording to the present invention. The timer means 26 measures the timeperiod during which the distance parameter of the synchronizationsignature must increase a predetermined number of times (the changesmust occur in succession) so that the situation would be interpreted asthe circulation of an outdated synchronization signature. Every time thedistance parameter increases within the time period measured by thetimer means 26 and the parameter describing the original timing sourcedoes not change, the decision means increments the counter 27. If thecounter reaches a predetermined threshold value within the time period Tmeasured by the timer means, the situation is interpreted as thecirculation of an outdated synchronization signature, thus leading tothe aforementioned recovery measures being started, which in practicemeans for example that (SOMS network) the node is forced to enter intothe state of internal timing, whereupon the decision means starts thetimer means 25 corresponding to the state of internal timing.Simultaneously with the starting of the recovery measures the decisionmeans 23 resets both the counter 27 and the timer means 26.

FIG. 8 shows a more detailed embodiment of the basic principleillustrated in FIG. 6, applicable for example in the SOMS network.Whenever a new synchronization signature is placed on the top of thepriority list (step 91), it is first examined whether the time Tmeasured by the timer means has expired (step 92). If it has, thedecision means starts the timer means 26 (step 93) and sets the counter27 back to its initial value (step 94). The node then continues itsnormal operation (95) checking again the time that is left in the timermeans when a new synchronization signature has been selected, the newnode outgoing signature being derived from this signature.

If the node decision means detects in step 92 that the time of the timermeans has not expired yet, it examines (step 92a) whether the newsynchronization signature originates from the same master clock as theold signature. In a SOMS network this can be performed for example insuch a way that the decision means compares the first part D1 of the newsignature to the first part of the node outgoing signature to see ifthey are identical. If they are, the decision means enters into step 92bin which it examines whether the distance parameter contained in thesynchronization signature has increased from its earlier value. In aSOMS network this may be performed for example by examining whether thesecond part D2 of the new signature is the same or bigger than thesecond part of the node outgoing signature. If it is, the distanceparameter has increased, whereupon the decision means steps the counter27 one step forward (step 92c). It is then examined (step 93c) whetherthe counter has reached its threshold value (or whether this value hasbeen exceeded, depending on how the threshold value is defined). If ithas, the node decision means starts the above-described recoverymeasures in order to eliminate the old signature from the network. Thecounter 27 and the timer means 26 are then reset (step 93a), and normaloperation is continued (step 95). The same procedures (resetting thecounter and the timer means, and transition to the normal operation) arealso performed if it is detected in step 92a that the newsynchronization signature does not originate from the same master clockas the old signature, and also if it is found out in step 92b that thedistance parameter has not increased from its earlier value.

As the description above illustrates, the timer means 26 and the counter27 are reset whenever the parameters of the signature do not correspondto the required "circulation criteria", i.e. the changes complying withthe criteria must be successive in order that the threshold value of thecounter 27 would be reached. If synchronization is selected on the basisof an incoming synchronization signature which is better than theprevious selected signature or in which the parameter describing theoriginal source deviates from that of the previous selected signature,the counter and the timer means are reset. They are also reset wheneverthe node is started.

In an actual realization of the method, the time period T of the timermeans 26 and the threshold of the counter 27 can be parametrized in adesired manner.

The recovery measures may also vary. It is for example possible toselect a fixed external source for a certain time period and to retainit for the time period, or the node may transmit a certain message tothe direction with which it is synchronized. Each node forwards thismessage to the direction with which it is synchronized. If a nodereceives its own message, it enters into a predetermined standard state,e.g. the state of internal timing.

According to a preferred embodiment of the invention, theabove-described check-up of the circulation situation is performed infull before the node outgoing signature is formed on the basis of thenew synchronization signature. This prevents a situation in which a nodetransmits a certain, in a way incorrect signature for a short while andthen enters for example into the transmission of the internal signature.The node outgoing signature is thus formed only in step 65 (FIG. 6) orstep 95 (FIG. 8), and the new received synchronization signature is usedin the formation only if it has first been discovered that there is noreason to start the recovery measures.

Even though the invention is described above with reference to theexamples according to the accompanying drawings, it is clear that theinvention is not restricted thereto, but it may be modified within theinventive idea disclosed above and in the appended claims. Even thoughthe SOMS system has been used as an example above, the solutionaccording to the invention is applicable in all similar systems wherethe synchronization signature contains in some form information aboutthe distance to the master clock of the network and about the originaltiming source. Furthermore, when the synchronization signatures on thepriority list are described above and in the appended claims, it isclear that the list may also comprises any other signature, for examplea port, on the basis of which the timing source can be identified,wherefore the wording of the claims should be understood to be generalin this sense.

I claim:
 1. A hierarchical synchronization method for a node in atelecommunications system employing message-based synchronization andcomprising a plurality of nodes interconnected by transmission lines,said method comprising:interchanging, with neighboring nodes, signalscontaining synchronization messages having a synchronization signature,said synchronization signature indicating the priority of the respectivesignal in the internal synchronization hierarchy of thetelecommunications system and containing a distance parameterrepresenting a distance to a master source of synchronization and amaster node parameter indicating an original synchronization source,synchronizing with the one of signals received from said neighboringnodes which has the highest priority in the internal synchronizationhierarchy, maintaining a synchronization list comprising, on the highestlevel, the synchronization signature of said signal with which the nodeis synchronized, monitoring said distance parameter and said master nodeparameter in said synchronization signature on the highest level in saidsynchronization list, changing over from a normal state to apredetermined state in order to prevent synchronization with signalshaving faulty synchronization signatures, when the value of saiddistance parameter increases a predetermined number of times within apredetermined monitoring period while said master node parameter remainsunchanged within said monitoring period.
 2. A method according to claim1, comprising:measuring said monitoring period with a timer means whichis started whenever the time period measured has expired and a newsynchronization signature is selected and placed on the highest level inthe priority list, and counting said successive times with a counter. 3.A method according to claim 2, comprising setting the counter to itsinitial value, when the timer means is started.
 4. A method according toclaim 3, comprising increasing the counter within said monitoring periodeach time the value of said distance parameter in the new signature hasincreased from the value of said distance parameter in the old signatureand when said master node parameter is the same as that in the oldsignature.
 5. A method according to claim 2, comprising:using internaltiming in said predetermined state, and resetting said timer means andsaid counter in response to the transition to said predetermined state.6. A method according to claim 1, comprising:forcing the node tosynchronize itself with an external source in said predetermined state,and resetting said timer means and said timer means and said counter inresponse to the transition to said predetermined state.
 7. A nodeequipment for a telecommunications system employing message-basedsynchronization and comprising a plurality of nodes interconnected bytransmission lines, said node comprising:several interfaces to which thetransmission lines to the neighboring nodes are connected, means forgenerating a synchronization signature to be transmitted between thenodes from a signal received at an interface, said synchronizationsignature indicating the priority of the respective signal in aninternal synchronization hierarchy of the telecommunications system andcontaining a distance parameter representing a distance to a mastersource of synchronization and a master node parameter indicating anoriginal synchronization source, means for comparing synchronizationsignatures received from the interfaces with each other, means forselecting the synchronization signature with the highest priority as asource of a node synchronization, means for forming a node outgoingsynchronization signature on the basis of the selected synchronizationsignature, means for counting the number of times in succession thevalue of said distance parameter received in a new synchronizationsignature received is higher than the value of said distance parameterin the selected synchronization signature while the master nodeparameter is unchanged, and means for changing the node from a normalstate into a predetermined state in order to prevent a selection offaulty synchronization signatures.